Depending on the field of use, motor systems are now provided with units which allow soft starting of the motor, in order to avoid jerky starting of the motor, for example when the maximum operating voltage is applied immediately. For this purpose, the motor is normally provided with a driver circuit which is driven by a control unit in a suitable manner, for example by way of phase-gating control, in order to ensure that the voltage which is supplied to the motor rises slowly during a switching-on phase. The driver circuit normally has power semiconductors which absorb a portion of the power provided to the motor, and convert this to heat losses.
In order to reduce the power loss in the driver circuit, the driver circuit is bridged after the switching-on phase, that is to say on reaching the maximum motor drive level, as a result of which the power loss in the driver circuit, that is to say in its power semiconductors, is suppressed. This considerably reduces the power loss during continuous operation, in particular at the rated motor load, thus making it easier to provide air-conditioning for the switchgear cabinet. Furthermore, this makes it possible to dispense with physically large cooling elements, thus allowing a compact device design.
When the driver circuit is bridged, the motor is normally driven with the maximum available voltage. When the motor is being operated in a partial load range, that is to say in a range in which the load applied to the motor is less than a rated load, the laws of physics mean that the current which is drawn by the motor does not decrease to the same extent. However, this results in higher heat losses than necessary in the motor, as a result of which more energy is required to operate the motor than is necessary. When the motor in motor systems in which it is not possible to bridge the driver circuit is operated in the rated load range, high losses generally occur in the power semiconductors in the driver circuit.
The document U.S. Pat. No. 4,577,604 discloses a control system for a fuel pump, in which the operating speed of the fuel pump is controlled in order to control the fuel pressure in a fuel supply system in an internal combustion engine. The control system comprises a main control circuit, which permanently connects a driver circuit to an electrical power supply, and an auxiliary circuit which is opened during normal operation. The main control circuit matches the electrical power supply to the driver circuit, in order to match the operating speed of the fuel pump either to a first maximum speed or to a second minimum speed. The auxiliary circuit establishes an electrical connection between the driver circuit and the electrical power supply when an increase in the fuel pressure in the supply line is demanded, and the air-fuel ratio is therefore detected as being above a predetermined value on the basis of previously selected operating parameters.